In fact, suburbs account for 50 percent of the nation's household carbon footprint, even though they are home to less than half of the U.S. population, according to a new UC Berkeley study. Their inhabitants drive more, consume more goods and use more electricity, gas and water, the research showed.

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The study was published last month in the journal Environmental Science and Technology, arriving at a time when urban populations are booming, here in the Bay Area and worldwide, and planners are trying to accommodate that increase in a sustainable way.

Unchecked growth could have consequences for the environment and personal health. As carbon dioxide and other greenhouse gases from human activities get trapped in the atmosphere, scientists expect the temperatures that rise as a result to worsen air quality, change the patterns of infectious diseases and make it harder to grow a stable food supply.

The analysis

The study analyzed 37 sources of data, including the census, weather, fuel prices, demographic information and household size to come up with estimates for the average carbon footprint of households in more than 31,000 ZIP codes across every state.

The resulting map shows a clear pattern of major urban areas with light carbon footprints surrounded by the heavy footprints of their suburbs.

For example, San Francisco's Hayes Valley neighborhood - ZIP code 94102, just west of downtown - generates 26 metric tons of carbon dioxide per average household annually. By contrast, households in ZIP code 94583, in San Ramon, generate a whopping 58 metric tons per year on average. Suburb-dwellers drive their cars longer distances and more frequently; typically have bigger houses that gobble up more electricity, water, natural gas and other fuels; and have higher incomes to buy more food and goods - all activities that drive up emissions.

Increasing population density in cities reduces emissions - but only to a point before the savings aren't that substantial and quality of life begins to suffer, said Kammen, who worked on the study with doctoral candidate Christopher Jones.

"The idea that your carbon footprint is lower if you live in the urban core doesn't mean everyone should be dumped into apartment buildings like Tokyo and live like an anonymous number," he said. "There are really interesting decisions and trade-offs we can get at by not just making everyone live in huge mega-blocks or spread out across a rural landscape."

Strategies to reduce emissions should be based on a region's unique needs and resources, Kammen said.

Parts of the Midwest, for example, largely use coal to generate electricity, which drives up emissions. California, on the other hand, has relatively low emissions associated with household electricity, but greater amounts from transportation. In the Bay Area alone, transportation is the source of more than 35 percent of emissions.

Much longer commute

Traffic has become familiar to Kelly Weekes, 33, who used to walk from her studio apartment to her job in downtown San Francisco. Now, every weekday, she commutes three to four hours between the condo in Concord she shares with her fiance and her new job in Daly City, which requires her to have a car on-site.

Weekes despises the drive, but says it was the more environmentally sustainable decision for her and her partner, who works near their home. "It made more sense for one of us to be driving," she said. "We live less than a mile from his job, and he's able to bike and walk."

The region expects to add 2.1 million people in the next 25 years, bringing the population to 9 million. Commutes could lengthen as rising housing costs drive residents from San Francisco, and as new residents move in droves to the more affordable Alameda and Santa Clara counties.

To ease pressure on the region's transportation systems, Plan Bay Area, the region's outline for development until 2040, calls for concentrating housing in neighborhoods within walking distance of public transit and amenities like grocery stores and restaurants.

That reflects an increasing desire of people of all ages to live in walkable communities, said Jeff Hobson, deputy director of TransForm, an Oakland group that advocates for public transportation.

"Having a low-carbon lifestyle is not just for hipsters," he said. "It should also be for soccer moms and NASCAR dads and Instagram teens."

Most suburb dwellers will still drive cars, said John Goodwin, a spokesman for the Metropolitan Transportation Commission, which developed Plan Bay Area with the Association of Bay Area Governments. But public transit will at least be "a viable option" for more of them, he said.

Weaning off cars

Plan Bay Area also specifies how nearly $300 billion in anticipated federal, state and local funds will be spent on maintaining and operating the current transportation network, and expanding transit, roadways and bridges. Under state law, the Bay Area must carry out this expansion while reducing per-capita carbon dioxide emissions from cars and light-duty trucks by 15 percent by 2040.

To make the air cleaner, agencies are trying to wean people off their cars.

One example is the Bay Area Bike Share program, which allows the public in five cities to rent bicycles. In addition, the Bay Area Air Quality Management District and the Metropolitan Transportation Commission plan to launch a commute-benefits program that may allow employees at large companies to pay for their public transit commutes with pretax dollars.

In the future, Kammen hopes that urban centers and their neighbors will achieve a sort of balance. "It's not that the urban core is good and the suburban core is bad," he said. "It's that infrastructure and planning are really critical."